1. Field of the Invention
The present invention relates to a glass-ceramic compound having a coefficient of thermal expansion that nearly coincides with that of quartz and to a a high-reliability of a flat-package for surface mounting of piezo-electric devices such as a quartz oscillator or a surface-acoustic wave resonator (SAW).
2. Description of the Related Art
When using such devices as quartz resonators as surface-mounted devices, as shown in FIG. 4, after affixing a quartz crystal 2 onto a base material 1 having alumina as its principal material, a cap 3 having alumina as its principal material is mounted thereover, after which hermetic sealing is done with a glass seal 4 having a low melting point. The sealed package 5 is mounted to a circuit board by means of, for example, the solder reflow method.
When sealing the package or when mounting these devices to a circuit board, the package is subjected to heat, and because the coefficients of thermal expansion of the package 5 and the quartz crystal 2 differ, after heating and cooling strain occurs in the quartz crystal 2. For this reason, the resonant frequency of the quartz crystal 2 changes, making it impossible to obtain the desired frequency characteristics.
As a countermeasure to this problem, a construction such as shown in FIG. 4, for example, which was described in "ELECTRONICS UPDATE" (p. 83 to p. 88 of Issue No. 4 in 1990) is used, in which the quartz crystal 2 is affixed to the base material 1 via a springy supporter 6, after which sealing is done.
Another method is that disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 2-105710, wherein the electrode leads 7 of the quartz crystal 2 are lead out to the same edge part of the quartz crystal 2, quartz crystal 2 being directly affixed to the base material 1 by fixing it to an electrode pad part 8 without an intervening supporter.
However, because with a quartz oscillator element such as noted above the coefficients of thermal expansion of the package 5 and the quartz crystal 2 differ from each other (for example, 70 to 80.times.10.sup.-7 /.degree.C. for alumina and 139.times.10-7/.degree.C. for the quartz crystal (Z long cut)), after heating and cooling, strain occurs, causing a change in the resonant frequency of the quartz crystal and preventing the achievement of the desired frequency characteristics.
Solving the above-noted problem by connecting the quartz crystal 2 to the package 5 via a springy supporter 6 is accompanied by an increase in the manufacturing cost and results in a package 5 with increased thickness.
In addition, alumina (Al2O3), which is the principal material of the package 5 has a sintering temperature in the range 1500.degree. to 1600.degree. C., so that when simultaneously heat-forming the wiring conductors within the package 5, it is necessary to use a high melting point metal such as tungsten (W) or Molybdenum (Mo) for the conductors.
Because the electrical conductivity of such high melting point metals is low, and because they cannot be soldered, it is necessary to provide a plating of Nickel (Ni) or Gold (Au). For this reason, a large number of steps and a high cost are involved in manufacture.
On the other hand, as one of other method to solve this conventional problems, Japanese Unexamined Patent Publication (KOKAI) No. 6-191887 is known.
This prior art discloses a glass-ceramic composite wherein 30 to 70% forsterite by weight are dispersed in crystallized glass and further it also discloses a method for making a flat package type piezo-electric devices comprising a quartz oscillator by forming the package made of the crystallized glass with mixing a plurality of specifically selected composing elements under specific mixing ratio, respectively.
However, in this prior art, the coefficients of thermal expansion of an element of the crystallized glass is at most around 120.times.10.sup.-7 /.degree.C. and thus there is a significant difference between the coefficients of thermal expansion of the crystallized glass and that of the quartz oscillator having the coefficients of thermal expansion of 139.times.10.sup.-7 /.degree.C.
Further, in a specific embodiment of this prior art, although crystallized glass portions each having the coefficients of thermal expansion of 129.times.10.sup.-7 /.degree.C. and of 135.times.10.sup.7 /.degree.C., respectively, the bending resistance of each thereof is relatively low and since each of the crystallized glass in this embodiments contains relatively large amount of sodium components therein, another problem will be raised in that it generally has deteriorated anti-moisture characteristic.
Accordingly, in this prior art, it would be difficult to produce a glass-ceramic compound having a coefficients of thermal expansion being substantially identical to that of quartz oscillators and having high-level of bending resistance and anti-moisture characteristic.